The present invention relates to an LCD (liquid crystal display) panel driving circuit. In recent years, the LCD has rapidly been widespread as a display unit for television and office equipment for ordinary home use. The reasons behind this are that the LCD is thin and lightweight as compared to a CRT and the display quality is not very inferior to that of the CRT can be obtained.
FIG. 1 is a schematic view showing the key portions of a conventional LCD panel driving circuit. The driving circuit comprises n units of selectors 11(1), 11(2), . . . , 11(n); n units of operational amplifiers 12(1), 12(2), . . . , 12(n) each operating as a buffer amplifier; and n units of output-polarity selection switches 13(1), 13(2), . . . , 13(n). Wherein n is a multiple of 2.
Of the selectors 11(1), 11(2), . . . , 11(n), for instance, odd-numbered selectors are dedicated to positive-polarity output, and even-numbered selectors are dedicated to negative-polarity output. Input into each of the selectors 11(1), 11(3), . . . , 11(nxe2x88x921) dedicated to the positive-polarity output are, for instance, 6-bit data for positive-polarity output and a positive tone voltage. Input, on the other hand, into each of the selectors 11(2), 11(4), . . . , 11(n) dedicated to the negative-polarity output are, for instance, 6-bit data for negative-polarity output and a negative tone voltage.
Of the operational amplifiers 12(1), 12(2), . . . , 12(n), half of them are operational amplifiers dedicated to positive-polarity output, and the remaining half are dedicated to negative-polarity output. Output voltage from the selectors 11(1), 11(3), . . . , 11(nxe2x88x921) for positive-polarity output respectively loaded into each of the non-inverted terminal of the operational amplifiers 12(1), 12(3), . . . , 12(nxe2x88x921) for positive-polarity output.
Output voltage from the selectors 11b, 11d, . . . , and 11n for positive-polarity output is respectively loaded into each non-inverted input terminal of the operational amplifiers 12b, 12d, and 12m for negative-polarity output.
The output-polarity selection switches 13(1), 13(2), . . . , 13(n) are connected to output pads 14(1), 14(2), . . . , 14(n) respectively. The output pads 14(1), 14(2), . . . , 14(n) are electrically connected to a LCD panel not shown herein.
Effects of an LCD panel driving circuit are explained below together with each switching operation of the output-polarity selection switches 13(1), 13(2), . . . , 13(n). However, it is assumed that k is an integer of 1 or more than 1 for convenience in description. When 2kxe2x88x921-th data D2kxe2x88x921 has a positive polarity, the data D2kxe2x88x921 is inputted into a 2kxe2x88x921-th selector.
The 2kxe2x88x921-th output-polarity selection switches are electrically connected towards positive polarity (the broken line shown in FIG. 1). Therefore, the positive-polarity driving voltage output from the 2kxe2x88x921-th selectors is output to 2kxe2x88x921-th output pads via the 2kxe2x88x921-th operational amplifiers and the 2kxe2x88x921-th output-polarity selection switches.
Here, 2k-th data D2k acquires a negative polarity and is inputted into 2k-th selectors. Therefore, 2k-th output-polarity selection switches are electrically connected towards negative polarity (the broken lines shown in FIG. 1). Therefore, the negative-polarity driving voltage output from the 2k-th selectors is output to 2k-th output pads via the 2k-th operational amplifiers and the 2k-th output-polarity selection switches.
Namely, the driving voltages of 2kxe2x88x921-th data lines acqure a positive-polarity driving voltage based on positive-polarity data D2kxe2x88x921, whereas the driving voltages of 2k-th data lines acquire a negative-polarity driving voltage based on negative-polarity data D2k. 
The data D2kxe2x88x921 and data D2k invert polarity at a prespecified cycle in the previous stages of the 2kxe2x88x921-th and 2k-th selectors. The data D2k-1 with negative polarity is input into the 2k-th selectors. The data D2k with positive polarity is input to the 2kxe2x88x921-th selectors. The 2kxe2x88x921-th output-polarity selection switches are electrically connected towards negative polarity (the solid line in FIG. 1). The 2k-th output-polarity selection switches are electrically connected towards positive polarity (the solid line in FIG. 1).
Therefore, the negative-polarity driving voltage output from the 2k-th selectors is output to 2kxe2x88x921-th output pads via the 2k-th operational amplifiers and the 2kxe2x88x921-th output-polarity selection switches. Each positive-polarity driving voltage outputted from the 2kxe2x88x921-th selectors is output to 2k-th output pads via the 2kxe2x88x921-th operational amplifiers and the 2k-th output-polarity selection switches.
Namely, the driving voltages of the 2kxe2x88x921-th data lines acquire negative-polarity driving voltage based on negative-polarity data D2kxe2x88x921, whereas the driving voltages of the 2k-th data lines acquire positive-polarity driving voltage based on positive-polarity data D2k. Therefore, a positive-polarity driving voltage based on the positive-polarity data D2kxe2x88x921 and a negative-polarity driving voltage based on the negative-polarity data D2kxe2x88x921 are alternately loaded to each driving voltage of the 2kxe2x88x921-th data lines at a prespecified cycle.
Further, a negative-polarity driving voltage based on the negative-polarity data D2k and a positive-polarity driving voltage based on the positive-polarity data D2k are alternately loaded to each driving voltage of the 2k-th data lines at a prespecified cycle.
The positive-polarity driving voltage based on the positive-polarity data D2kxe2x88x921 and the negative-polarity driving voltage based on the negative-polarity data D2kxe2x88x921 are opposite in polarity, but has the same level. The same is true with respect to the negative-polarity driving voltage based on the negative-polarity data D2k and the positive-polarity driving voltage based on the positive-polarity data D2k. 
The reason why AC driving is performed such that a positive-polarity driving voltage and a negative-polarity driving voltage are alternately loaded to an identical pixel at a prespecified cycle as described above is because inconvenience of degradation in a liquid crystal under the situation where a voltage with the same polarity is kept on being loaded to an identical pixel should be avoided. However, screen flicker occurs due to AC driving. To suppress the flicker, in the LCD, driving voltages opposite in polarity are loaded to adjacent data lines, and voltages opposite in polarity are loaded to adjacent pixels.
In the above-described conventional type of LCD panel driving circuit, each driving voltage of the 2kxe2x88x921-th data lines is prepared with each output voltage from the 2kxe2x88x921-th operational amplifiers and each output voltage from the 2k-th operational amplifiers. Further, each driving voltage of the 2k-th data lines is also prepared with each output voltage from the 2kxe2x88x921-th operational amplifiers and each output voltage from the 2k-th operational amplifiers.
Therefore, even when there are offset voltages in the 2kxe2x88x921-th and 2k-th operational amplifiers, an offset difference does not occur between each driving voltage of the 2kxe2x88x921-th data lines and each driving voltage of the 2k-th data lines. Similarly, even when there are offset voltages in the 2k+1-th and 2k+2-th operational amplifiers, an offset difference does not occur between each driving voltage of the 2k+1-th data lines and each driving voltage of the 2k+2-th data lines.
However, when an offset voltage in the 2kxe2x88x921-th operational amplifier and that in the 2k+1-th operational amplifier are opposite in polarity or an offset voltage in the 2k-th operational amplifier and that in the 2k+2-th operational amplifier are opposite in polarity, even if identical-tone display is performed, a large voltage difference occurs between each driving voltage in the 2k-th data lines and each driving voltage in the 2k+1-th data lines. Therefore, there is a problem such that unevenness in brightness and longitudinal streaks may appear on the screen on display of identical tone.
The occurrence of offset voltages in operational amplifiers is caused by variations in the manufacturing process of transistors. Therefore, in the conventional technology, variations in the manufacturing process are decreased by increasing the area of a transistor forming a current mirror circuit so that an offset voltage in an operational amplifier can be made smaller. However, this technology has a weak point that the size of the LCD panel driving circuit increases.
It is an object of the present invention to provide an LCD panel driving circuit for making the panel perform high-quality display without an unevenness in the brightness and longitudinal streaks on a screen without the necessity of increasing a scale of the circuit.
In the present invention, the tone voltages supplied to any two adjacent output terminals among a plurality of output terminals which are arranged on one line are always output from two adjacent buffer amplifiers among a plurality of buffer amplifiers which are arranged on one line.
Further, the present invention comprises j units of first-polarity selectors, j units of second-polarity selectors, j units of data-line selection switches, j units of first-polarity buffer amplifiers, j+1 units of second-polarity buffer amplifiers, and 2j units of output-polarity selection switches. Output voltage of each of the 2j units of selectors is supplied to each of the 2j data lines via the data-line selection switches, the buffer amplifiers, and the output-polarity selection switches.
Further, the first-polarity selectors are connected to the corresponding first-polarity buffer amplifiers. Each of the first-polarity buffer amplifiers is connected to either one of a first data line and a second data line which are adjacent to each other via the corresponding output-polarity selection switch. Each of the second-polarity selectors is connected to either one of the corresponding pair of second-polarity buffer amplifiers via the corresponding data-line selection switch.
Of the pair of second-polarity buffer amplifiers, one of the buffer amplifiers is connected to either one of the first data line and a third data line adjacent thereto via the corresponding output-polarity selection switch. The other one of the buffer amplifiers is connected to either one of the second data line and a fourth data line adjacent thereto via the corresponding output-polarity selection switch. The data-line selection switches and the output-polarity selection switches are concurrently switched at a prespecified timing.
With the above-described configuration, output voltage of the first-polarity buffer amplifier and the output voltage of one of the pair of second-polarity buffer amplifiers are supplied to the first data line. Further, output voltage of the first-polarity buffer amplifier and the output voltage of the other one of the pair of second-polarity buffer amplifiers are supplied to the second data line.
Further, output voltage of the other one of the pair of second-polarity buffer amplifiers and the output voltage of a first-polarity buffer amplifier different from the first-polarity buffer amplifier connected to the first or second data line are supplied to the third data line. Similarly, output voltage of one of the pair of second-polarity buffer amplifiers and the output voltage of a first-polarity buffer amplifier different from the first-polarity buffer amplifier connected to the first, second, or third data line are supplied to the fourth data line.
Namely, a common buffer amplifier is connected to any adjacent data lines. Therefore, it is possible to prevent occurrence of a large difference in driving voltages for performing identical tone display between any adjacent data lines, thus appearance of unevenness in brightness as well as of longitudinal streaks on a screen can be prevented on display of identical tone.
In addition, with the above-described configuration, when an operational amplifier is used as a buffer amplifier, the need to decrease the offset voltage in the operational amplifier by increasing the area of a transistor forming a current mirror circuit is eliminated, thus the scale of the LCD panel driving circuit can be reduced. Resultantly, by using this LCD panel in a display, the display can be downsized.